Tyres for road-going vehicles are typically provided with a complex pattern of grooves, known as tread, for displacing water from between the tyre and a road surface in order to improve traction in wet conditions. Because of the safety importance of tyre tread, national laws typically prescribe minimum tread depths for the tyres of road-going vehicles. For example, the tyres of a domestic car in the United Kingdom are required to have a minimum depth of the tread of 1.6 mm in a continuous band throughout the central 75% of the tread width and around the complete circumference of the tyre. Heavier vehicles in the UK have similar requirements but to a minimum depth of 1 mm.
Tread depth, however, is difficult to measure accurately. Often only an unreliable, qualitative visual inspection may be performed. Quantitatively, a manual tread depth gauge, comprising a simple sliding ruled plunger may be used to determine an approximate tread depth at a single point on the tyre's surface. Manual devices of this type suffer from a wide variation in readings from operator to operator, and they cannot practicably be used to determine the tread depth across and around most or all of the tyre's rolling surface.
As purchasing and fitting replacement tyres can form a significant part of the total cost of running a vehicle, premature replacement can be regarded as wasteful. This is especially true for fleet operators, such as a bus and haulage company, who have large numbers of tyres to replace each year. Conversely, continuing to use tyres that do not satisfy legal or commercial minimum tread requirements may be illegal and can compromise vehicle safety. Thus, a system for easily monitoring the wear of a tyre is required. Since removing the wheel from a vehicle is a time-consuming operation, it is desirable that in situ tread depth measuring can occur (i.e. with the tyre still on the vehicle). Having to drive the vehicle onto a “rolling road” in a garage or depot is often undesirable due to it being time-consuming (potentially requiring both a driver and an assistant) and space-consuming.
Rather, a portable apparatus for in situ measuring is desired, which can be used by a single operator on stationary vehicles for a thorough inspection of the vehicle's tyre surface. In this way, no additional space in a garage or depot is required, and vehicles can be efficiently parked, and the drivers dismissed, before tyre monitoring commences.
U.S. Pat. No. 5,895,845 (Buerger) describes a mobile device for measuring the tread depth of a tyre at a single point. The device is arranged so that it can be manually swept along a single line across the width of the tyre in the axial direction. However, scanning only along a single line will not detect excessive wear or erratic bald spots on the tyre's surface; nor can it provide information for checking that the tread forms a continuous band around the circumference of the tyre. Since tyre wear is often not, in practice, circumferentially uniform, this is a significant shortcoming when a thorough inspection of the tyre is required. This device is not able to confirm compliance with legal minimum tread requirements with a high degree of confidence.
U.S. Pat. No. 6,789,416 (Tracy et al.) also teaches a device for measuring a tyre tread profile by automatically scanning a laser spot along a line across the width of the tyre in the axial direction. It also therefore suffers the same shortcomings in this regard as the device of U.S. Pat. No. 5,895,845. Furthermore, it employs a motor-driven toothed belt to move a range finder along the length of a rod while measurement is being taken. Such reliance on moving parts is undesirable.
U.S. Pat. No. 5,987,978 (Whitehead) describes a handheld device provided with guide rollers allowing the device to be rolled around the circumference of the tyre, and having an inclined, facetted mirror, with a saw-tooth profile, spanning the width of the tyre, in order that light from a single light source located away from the tyre may directed at the tyre. However, the Applicant has calculated that in order to transfer a useful amount of light onto the tyre surface, such a mirror arrangement would need to be inclined close to 45 degrees, and therefore result in a device at least as high as the tyre is wide, which would make it awkward to handle and far too big to fit within the space between the tyre surface and a typical vehicle's wheel arch (a gap of typically up to 100 mm in places for a commercial vehicle). At shallower angles, such as are suggested in the drawings, the facetted mirror would direct only a line of spots of light onto the tyre's surface rather than a continuous line of light, thus yielding poor coverage of the whole width of the tyre's rolling surface.